(i) Field of the Invention
This invention relates to a method and apparatus for continuous casting of molten lead alloys as strip and, more particularly, to high speed continuous casting of thick lead alloy strip.
(ii) Description of the Related Art
Battery electrodes meant for service in industrial, motive power, and/or telecomm batteries are typically made using a book moulding procedure, i.e. gravity casting. Book moulding is a means to solidify molten lead directly into a thick battery electrode, wherein the molten lead is fed into a steel mould, solidified, and released.
Thick positive battery grids made by gravity casting methods have a porous and non-uniform micro-structure which promotes corrosion, can be subject to grid growth, and cause high water loss in a battery. All these characteristics shorten the battery life. The gravity casting method, however, is the only method that is used on a commercial scale to make positive low antimony grid electrodes.
U.S. Pat. No. 5,462,109 granted to Cominco Ltd. (Now Teck Metals Ltd.), incorporated herein by reference, discloses a method and apparatus for continuously casting a lead alloy strip, including antimony strip. The strip is cast on a chilled, pebbled casting surface of a rotating drum from a pool of the molten metal contained in a tundish having a graphite lip insert seated therein cooperating with the casting surface adjacent to the tundish to form and contain the pool of the molten metal. A preferred lead alloy is an antimony-lead alloy containing up to 4.0 wt % antimony which is cast into strip and is subjected to a heat treatment to provide integrity and strength necessary to permit subsequent production of expanded mesh battery grids. The battery grids produced by this method have improved electrochemical properties such as corrosion resistance and resistance to growth. However, although thin and narrow antimony-lead alloy strip can be produced at low speeds of 36-38 feet/minute in a thickness in the range of 0.02″ to 0.06″ and in widths up to five inches, it has been found that both thin and thick low antimony lead strip continuously cast on a commercial high speed basis for use as positive electrodes suffered from the formation of longitudinal cracks in the direction of casting during the solidification process particularly at increased casting speeds.
It is a principal object of the present invention therefore to provide a method and apparatus for continuously casting antimony lead alloy strip, particularly thick antimony lead strip, having up to and in excess of 5 wt % antimony, for industrial use, having an acceptable fine grain structure, essentially no porosity and high corrosion resistance.
It is another object of the invention to provide a method and apparatus for casting wide lead alloy strip in widths up to 20 inches which can be readily controlled for desired strip thickness from thin to thick strip ranging in thickness up to and above 0.185 inch and which allows for a wide selection of lead alloys, including lead alloys of antimony and calcium.
A further object of the invention is the provision of a method and apparatus which permits continuous high speed commercial casting of lead alloys into strip suitable for producing electrodes for heavy duty, industrial, motive power, telecomm, renewable energy, uninterruptible power supply and the like batteries.